1. Field of the Invention
The invention generally relates to GaN boule growth from a liquid melt using a GaN seed wafer, to produce a GaN boule yielding wafers suitable for fabrication of microelectronic and optoelectronic devices.
2. Description of the Related Art
GaN is a promising wide bandgap semiconductor material for optoelectronics and power electronics. Realization of its full potential is however limited by a lack of suitable bulk GaN substrates. Because GaN has a high melting temperature and a very high decomposition pressure at the melting point, bulk crystals of nitrides cannot be grown with conventional methods such as Czochralski or Bridgmen growth from stoichiometric melts.
Porowski et al disclosed a method of growing bulk GaN at high nitrogen pressure (S. Porowski and I. Grzegory, J. Cryst. Growth, Vol 178, 174 (1997)). Metallic gallium reacted with gaseous nitrogen to form gallium nitride crystals at the surface of the gallium melt. A temperature gradient was provided in the reactor vessel, resulting in supersaturation of nitrogen atoms in the cooler region of the reactor and growth of gallium nitride crystals. Small crystals of GaN were grown at a nitrogen pressure as high as 20000 atmospheric pressure and a temperature as high as 2000 K. However, the growth rate was small and long growth time of 60–150 hours was needed in order to grow crystal platelets of 6–10 mm in length and ˜0.1 mm in thickness. The small size of the crystal grown by this technique is not suitable as substrates for electronic and optoelectronic applications.
Inoue et al disclosed another method of growing bulk GaN at high nitrogen pressure and at high temperatures (Inoue et al, J. Cryst. Growth, Vol. 229, 35 (2001), and Phys. Stat. Sol. (a) Vol. 180, 51 (2000)). The difference between Inoue's technique and Porowski's technique is that Inoue uses nitrogen over-pressure to produce supersaturation of nitrogen in the gallium melt and formation of gallium nitride crystals in the gallium melt. Similar to Porowski's technique, Inoue's technique produces small GaN crystal platelets (<10 mm in length), and not suitable as substrate for commercial device applications. Furthermore, the high temperature and high pressure needed for the crystal growth provide additional challenges to scale up the size of the crystal with these techniques.
Single crystal GaN can also be grown with a so-called sodium flux method (see, for example, Aoki et al, J. Cryst. Growth, Vol. 218, 7 (2000) and DiSalvo et al, U.S. Pat. No. 5,868,837). DiSalvo et al disclosed a method of synthesis GaN by thermally decomposing sodium azide in a closed reactor zone (autoclave) containing metallic gallium. The autoclave was heated to 600–800C., where sodium azide decomposed to gaseous nitrogen that pressurized the system. However, the crystal size was small, in the millimeter range, even for growth of several days. Aoki et al disclosed a similar method of synthesizing GaN by reacting gaseous nitrogen with a metallic mixture of gallium and sodium at moderate temperature and pressure. At a growth temperature of 750C., a growth pressure of about 50 atmospheric pressure, and sodium molar ratio of 0.6, GaN crystal platelets of size in millimeter ranges were produced after 200 hours of growth. The small crystal size and slow grow rate limit the application of this growth technique.
Ketchum et al disclosed a method of growing gallium nitride crystals in supercritical ammonia using GaN powder as feedstock (D. R. Ketchum, J. W. Kolis, J. Crsyt. Growth, Vol 222, 431 (2001)). The feedstock was transported to form single crystals through the use of mineralizer. However, largest crystal obtained was only about 0.5 mm, which is not suitable as substrates for electronic and optoelectronic devices.
GaN substrates based on hydride vapor phase epitaxy (HVPE) are currently being developed, but such substrates suffer from various process-related deficiencies that have hindered their development and commercial availability. HVPE is a vapor phase growth process. Vapor phase growth is not an equilibrium growth technique and typically generates a significant level of defects, but has the advantage that the process is able to be carried out at relatively low temperature levels (e.g., ˜1000° C.).
The art is in need of improved low-cost processing techniques capable of forming large size wafers of GaN suitable for fabrication of optoelectronic and microelectronic devices.
Such large size wafers in turn require correspondingly sized boules, i.e., bulk masses of GaN from which multiple wafers can be derived, as for example by cutting with wire saws, blade saws, laser cutters, cleaving, etc.
Unfortunately, GaN boules cannot be grown with conventional boule growth methods utilized for forming single crystal wafer source bodies of materials such as silicon, because GaN decomposes before melting and a very high equilibrium pressure is necessary for GaN formation from Ga melt at the growth temperature. At the moderate pressures desired for commercial boule growth, the prior art has failed to provide a commercially viable method for forming large diameter (>10 mm diameter) single crystal boules of GaN.